1. Field of the Invention
The present invention relates to a method for making a color transform for an output device, the color transform representing a relation between a first and a second color space, each color space comprising color points for defining a color, each color point comprising a number of color channel values, the color channel values of the color points in the second color space controlling one or more colorants in the output device. The present invention further relates to a computer program product comprising instructions for executing the invented method.
2. Description of the Related Art
Reproductions of images involving color are often sensitive to the amount of colorants that are used to render an image on a receiving medium. Furthermore, the properties of the colorants, comprising both coloring material, such as dye or pigment, and a carrier, such as resin, water or another solvent, the properties of the receiving medium, such as paper or polymer film, and the applicating conditions, such as the temperature and humidity of the environment, all affect the output colors of a color imaging device, such as a printer. Colorants are applied either by varying a quantity of material emitted on a predefined position of the receiving medium, or by varying a number of fixed quantities of material around a predefined position, both identifiable as an adjustable colorant amount for an image position, controlled by a value of a control variable, sometimes referred to as a code value. If, for example, the colorant is ink, a colorant amount may be adjusted by varying the volume of an ink drop on a predefined position, or by the amount of ink drops with a fixed volume around this predefined position. Also combinations of these two variations are possible.
A single control variable, or a single color channel, is sufficient for an output device for monochrome reproductions, but for full color reproductions at least three color channels are necessary and most commonly, four color channels are used. An output device applies at least as many colorants as the number of available color channels. Each control variable in a color channel may be used for controlling a single colorant, indicating a value in a range from a minimum amount of colorant, most often equivalent to no colorant, to a maximum amount of colorant. A control variable may also be used to control more than one colorant, e.g. in the case of a combination of a light, or diluted, and a dark, or concentrated, colorant for varying an output color. A separation of the control variable to control the individual amounts of the two colorants is made within the output device. For color controlling purposes, such as by an external print controller, only the available color channels are relevant.
A color imaging device prints a digital image that is derived from print job data. A print job that is sent to the imaging device is first interpreted by a print controller, thereby converting colors as defined in the print job in a first color space to color channel values of a color point in a second color space, the color channel values being appropriate for obtaining a color by the imaging device corresponding to the defined color in the print job. In general, in the processing of digital images, colors may have to be converted using a predefined color transform, that represents a correspondence relation between two color spaces. Each color space comprises color points having a number of color channel values for defining a color. In a color space, a gamut is the set of colors that is available by a combination of color channel values in a color point.
Digital images use different combinations of channels for defining a color of image elements, such as pixels. The channels in a color imaging device, that is configured to make a reproduction of a digital image, may or may not correspond to the channels of an input image. An example is the common use of the three channels R, G, and B, also known as a red, green and blue channel, that define a color for each pixel of a raster image, and the four channels C, M, Y, and K, also known as a cyan, magenta, yellow and black channel, that are used to control the application of colorants in a printer. Each of these combinations of color channels is associated with a customary scheme for color generation by mixing the various channels. In the case of RGB-channels, additive mixing is usual, leading to a white color when all the channels have their maximum value, whereas in the case of CMYK-channels, subtractive mixing is conventional, leading to a black color when all the color channels have their maximum value. In order to accommodate the various sets of color channels, digital print systems comprise a conversion module for converting the channel control values of each pixel of the input image into channel control values that are applicable in the imaging, or output, device. This conversion module uses a color transform based on calibration data for characterising the input color space of the digital image and the output color space of the imaging device. In the calibration process, a color is established in its device dependent set of control values and related to a color definition in scientific, device independent variables, such as CIE XYZ or CIE L*a*b. An often used model for color matching is the combination of two characterising color profiles, one for the input side and one for the output side of the conversion, as prescribed by the International Color Consortium (www.color.org). These profiles are linked through a profile connecting space, such as the CIE XYZ or CIE L*a*b* color space, thereby obtaining a conversion table, also known as a device link or color transform, for the conversion module. The size of the conversion table is not fixed and the conversion module is capable to perform the necessary interpolations to convert input values that are not on a grid point of the table. Each profile defines a gamut, which is formed by the colors that may be represented by the color points in the device dependent color space. In general, a gamut is the set of colors that may be defined in a color space. A gamut may be limited by a predefined range for the color channels, such as e.g. by the limited range of 8-bit digital values. For colors outside the gamut, an alternative color point may be used. This alternative color point is also related to another color, a color within the gamut. The process for establishing a relation between a color outside the gamut and a color point that represents a color within the gamut is called gamut mapping.
A color profile, and in general a color transform, may be made by associating color channel values for an input or output device and their corresponding colors with their device independent color values in a black-box analysis. This means that the further processing of the color channel values with a device remains hidden during the making of a color profile. For a scanner, this comes down to determining the response in the color channels of the scanner to a number of color areas on a test chart. For a printer, this comes down to printing a test chart with a number of areas, each area having pixels with stimulant color channel values, on a representative medium and optically measuring the output colors of these areas. No further analysis of the color filters in a light sensitive element of the scanner or of the application of colorants in the printer is made. A resulting profile defines a relation between color channel values of the output device, and corresponding colorimetric values, in both conversion directions.
Not all media that receive colorants for the reproduction of an image in a printer, are capable of digesting large amounts of colorants. For this reason many profiling methods offer the possibility to limit the total ink coverage (TIC) or the total area coverage (TAC). This feature may also be used to reduce the amount of ink for economic reasons. For example, in a four channel print process, each channel is controlled from 0% to 100%, or from 0 to 1. If a color point comprises 100% for each channel, this results in a total area coverage of 400%. In many color management systems, it is possible to limit this value to a lower value, e.g. 250% or 320%, to prevent exceeding the technical limitations of the print process or to make a less expensive print by using less ink in high coverage areas, in dependence on the receiving medium and other process parameters. A limitation of the total coverage affects the gamut as, in addition to colors for which no color point exists, also colors for which the color point exceeds the established TAC limit, are excluded from the gamut. Therefore, the gamut shrinks by using a TAC limit.
It has been found that, by establishing a TAC limit, also the shape of the gamut changes relative to the shape of the gamut without a TAC limit. This has a considerable effect on the performance of gamut mapping for out of gamut colors, since colors outside the gamut without a TAC limit will often be mapped on the contour of the gamut. This is the part of the gamut where most of the colors transgressing the TAC limit are found. The colors on the gamut boundary are selectively eliminated, depending on the sum of the color channel values of the color point that produce these colors. Only colors that result from combinations of colorants for which the sum of the color channel values is lower than the TAC limit, are maintained and only these colors will be available for mapping out of gamut colors to.
A method for profiling, or more generally, a method for making a color transform, may be used for many different print processes or image processing purposes in which color channel values lead to colors in different ways. However, it has been found that in most processes, the use of a TAC limit leads to a concave gamut shape, wherein the parts of the gamut in which colors are composed of a single colorant or a few colorants stand out relative to colors using all colorants, whereas for many gamut mapping algorithms a convex shape is preferred.
Hence, a problem exists in controlling the volume and the shape of a gamut in a method for making a color transform, that relates different color channel values to each other, when a limit is imposed that restricts the available color points in the output color space.
An object of the present invention is to provide a method for making a color transform wherein control means are available for influencing the volume and the shape of the gamut in the output color space.